After administration of the serotonergic antidepressant citalopram (CIT) to Beagle dogs, the dogs may experience severe convulsive attacks in relation to the considerably higher plasma concentrations of the metabolite didesmethyl-CIT (DDCIT), when compared to those in humans medicated with CIT. This pilot study aimed at determining the role of cytochrome P-450 (CYP450) isozymes in the in vitro metabolism of CIT to desmethyl-CIT (DCIT), and of DCIT to DDCIT in the liver microsomes of a single Beagle dog. Incubations with racemic CIT or DCIT reveal a high-affinity enzyme with K _m between 0.3 μM and 1.4 μM for S- and R-DCIT and S- and R-DDCIT productions, respectively. In comparison to human enzymes, the intrinsic clearance values of this high-affinity enzyme are between 15 μl/(min × mg of protein) and 52 μl/(min × mg of protein), i.e., very high. In vitro experiments with inhibitors suggest that CYP2D15, which shows an analogy with human CYP2D6, is by far the main CYP450 isozyme involved in the production of DCIT and DDCIT, whereas CYP3A12 and CYP2C21/41 showed a weak implication. These observations partly explain why, in humans, the plasma concentrations of the toxic DDCIT are considerably lower than those observed in dogs, after administration of CIT.